A conventional projection television (TV) may be either a liquid crystal projection TV or CRT projection TV depending on image generations. As to the CRT projection TV, its projection can be classified as either a front projection or rear projection. The CRT projection TV comprises three monochromic CRTs for producing images of red (R), green (G), and blue (B) colors thereon respectively. The images of RGB are then amplified by a projection lens unit prior to projecting on a front or rear surface of a screen. As a result, a color image is generated on the screen as the images of RGB combined together.
Referring to FIG. 1, a schematic drawing of the typical front projection type CRT projection TV is shown. The lighted traces produced by each of three monochromic CRTs (consisting of a red CRT a green CRT and a blue CRT) 11 are amplified by a projection lens unit 12 prior to projecting on a reflective screen 13. The amplified lighted traces of RGB are then reflected from the reflective screen 13. As a result, a color image is generated on a TV screen for viewing as the reflected, amplified lighted traces of RGB combine.
Referring to FIG. 2, a schematic drawing of the typical rear projection type CRT projection TV is shown. The images produced by each of R, G, and B CRTs 21 are amplified by a projection lens unit 22 prior to projecting on at least one reflective mirror 23. The amplified images of RGB are then reflected to a transmitting screen 24 from the reflective mirror 23. As a result, a color image is generated on the transmitting screen 24 for viewing as the reflected and amplified images of RGB being combined together on the big screen.
As to the typical CRT projection TV of either the front projection or the rear projection, images produced by each monochromic CRT are required to be amplified by a projection lens unit prior to projecting on a screen spaced apart from the projection lens unit by a distance. For achieving this purpose, the images produced by each monochromic CRT should have an acceptable brightness. Otherwise, energy of electrons projected from the monochromic CRT may consume considerably due to prolonged projection path of electrons and an excessive number of reflections and/or projections, resulting in a low brightness and hue of color image produced on the reflective screen 13 or the transmitting screen 24.
For solving the above problem, designers and manufacturers of typical CRT projection TVs adopt a technique of applying a large current to an electron emission source (e.g., cathode) of each monochromic CRT. As a result, beams of high energy are generated for significantly increasing brightness of image produced on the reflective screen 13 or the transmitting screen 24. Such technique can substantially solve the problem of low brightness and hue of image as experienced in the prior art. However, the prior art still suffered from a disadvantage. For example, the number of electrons in a single beam may be increased significantly due to large current applied to the single electron emission source of each monochromic CRT. However, the beam spot size will increase gradually toward the screen of each monochromic CRT due to repelling of the charges of the same polarity. To the worse, the corona may degrade significantly on the screen. An improvement of the above problem may be made by either modifying a focusing lens or common lens in an electron gun of each monochromic CRT or increasing diameter or depth of a neck portion of each monochromic CRT. However, such improvement is very limited because the diameter of the CRT made of glass is fixed according to its specifications. Therefore, it is impossible to limitless enlarge a main lens of the electron gun. As an end, an optimum beam spot size is impossible to obtain. This is why the undesired low resolution and poor focusing quality of the typical CRT projection TV of either the front projection or the rear projection cannot be improved significantly despite years research.
Thus, it is desirable to provide an improved electron gun of monochromic CRT in order to overcome the above drawbacks of the prior art.